The invention relates to improvements in roller presses and particularly to high pressure roller presses capable of interparticle crushing.
In the development of roller presses, a unique improvement in the crushing of pulverulent material is disclosed in U.S. Pat. No. 4,357,287, Schoenert. Utilization of the interparticle crushing concept is disclosed in such patents as Beisner et al U.S. Pat. No. 4,703,897. A feature of this concept is that new and unforeseen results are attained in the utilization of unusually high nip pressures, as described in the aforementioned patents and comminution in accordance with this method creates incipient cracks in the particles so that the comminution is completed by breaking up the resultant scabs that are produced and such breaking up is accomplished with relatively low energy requirements. This new procedure has presented operational and functional problems not present in ordinary roller mill grinding and the present invention relates to a solution of certain of the problems and a discovery of an apparatus and a method of control of a roller press to obtain improved pressing with greater uniformity and uniform and improved energy consumption.
In a two roller machine of the type referred to, two oppositely driven rollers are separated from each other by a nip and at least one is constructed as a movable roller supported on pillow blocks. A force is applied to those pillow blocks by hydraulic cylinders and pistons. The hydraulic cylinders and pistons are part of a hydro-pneumatic system to apply pressing power to the rollers and include pressure boosting and pressure relieving valves to control the hydraulic fluid for the nip force. The arrangement also includes sensors for measuring the hydraulic fluid to thereby obtain a measurement of the nip force. The arrangement also contains measuring means for measuring the distance between the supporting pillow blocks at the ends of the rollers which will afford a measurement of nip width. A product delivery stack is arranged above the nip and contains a metering gate for controlling the feed of pulverulent material to the nip.
The two roller press with the pressing between them receives granular or pulverulent solid material which is subject to a pressure stressing in the nip. In one form, one of the two rollers is usually fashioned as a fixed roller which is supported directly against an end wall of the machine frame. The other roller is a movable roller supported at its end on movable pillow blocks against which the hydraulic cylinders of a hydro-pneumatic system apply force to obtain the nip force.
In a roller press of this type, shown for example in European Patent 0 084 383, individual particles of grinding stock are drawn into the nip by friction and are mutually crushed in a product bed. That is, material is compressed between the roller surfaces at an extremely high pressure to operate in accordance with interparticle crushing- This high pressure leads to partial product particle destruction in the grinding stock and creates incipient cracks in the insides of the particles. The particles are formed as agglomerates, called scabs, that are deagglomerated or disintegrated with a relatively small energy outlay.
In order to obtain optimum pressure stressing of the granular material, it is highly desirable to maintain operational parameters uniform. For example, the parameters are the force or roller pressing power in the nip and the nip width, and once these have been set at an optimum relationship, they should be maintained as constant as possible. Variations of the nip width that occur will be as a result of modifications of the charging stock. For example, modification of the input granulation can result in nip width. German Published Application 35 35 406 proposes a variation in the quantity of input stock for controlling the nip width and regulation in the material delivery can also be accomplished by a change in the height of the material column that loads the rollers at the intake of the nip which will influence the draw-in behavior of the nip.
One disadvantage of this operating arrangement is that where a constant roller pressing power and a constant nip width is used, the two roller machine cannot react to changes in material circulations that occur when the press is connected to additional machines such as a sifter, a classifier, or a ball mill.
It is accordingly an object of the present invention to create an improved two roller press and method of operating which makes it possible that optimum pressure stressing is always guaranteed even with substantial fluctuations of throughput or draw-in behavior of the nip and with fluctuations of the nip width resulting therefrom.
A further object of the invention is to provide an improved apparatus and method wherein nip force and nip width are continually measured and are maintained in predetermined relationship, preferably as a linear relationship.
A still further object of the invention is to provide an improved roller press and method of operating wherein improved more uniform interparticle crushing is obtained and ensured by continual control of the operating parameters of the press.